Scientific Objective To study the intraembryonic regulation of chorionic gonadotropin during the peri-implantation period of early pregnancy. Strategic Objective To determine the viability of embryos, the causes of early embryonic loss, and the regulation of implantation in primates (including humans). Chorionic gonadotropin is a hormone normally specific to pregnancy in primates, necessary for implantation and support of the corpus luteum of early pregnancy. We are testing the hypothesis that this hormone is essential for embryo-maternal attachment and that it may have physiological functions in embryonic differentiation. Studies are under way in rhesus and marmoset monkeys -- Old and New World species respectively. The timing of embryonic development was shown to be similar in tubal stages, with the embryo entering the uterus by day 4 after ovulation. Embryo attachment and implantation commenced on days 8 to 9 in the rhesus and on days 11-12 in the marmoset. Studies of implantation in vivo and in vitro showed that CG secretion was detectable 3 to 4 days after attachment in the peripheral circulation, but commenced at the time of embryo attachment in vitro. Recently we showed in the rhesus monkey that (i) low levels of CG are secreted from blastocyst stages, these levels increasing exponentially once attachment in vitro is achieved, and (ii) GnRH is also secreted by the preimplantation embryo, with levels increasing steadily through the period of study. In the past year we tested a possible functional relationship between GnRH and CG. Embryos were incubated with agonist and antagonists to GnRH. The results indicate a disruption of embryonic development, implantation and CG secretion showing that GnRH may be involved directly or indirectly in the regulation of CG secretion. Key words chorionic gonadotropin, implantation, embryonic loss, gonadotropin releasor hormone